An essential element in the design of flight vehicles is the prediction of aerodynamic drag. The aerodynamic drag includes form drag and skin friction drag. The form drag is associated with pressure changes that occur as the fluid flows around the vehicle in accordance with a shape of the vehicle. The skin friction drag arises from the friction of the fluid against the “skin” of the vehicle. The aerodynamic drag and hence the skin friction drag is also important for other types of vehicles which move through a fluid, such as trains, cars and submarines.
The skin friction drag arises from the interaction between the fluid and the skin of the vehicle. The skin can refer to the irregular shape of the wetted surface of the vehicle. For a given shape of a flight vehicle and specified flight conditions, the skin friction drag will vary depending on the surface finish used to construct the skin of vehicle. For example, the skin friction drag will be different for a polished metal skinned vehicle versus a vehicle with a skin with a sand paper like finish.
The interaction between a fluid and a surface finish that produces the skin friction drag can be characterized by a skin friction coefficient which is a ratio of the local wall shear stress divided by the free stream dynamic pressure. The skin friction coefficient is a function of the surface finish properties and the flow conditions. In particular, if the surface finish is anisotropic, the skin friction coefficient varies according to a flow direction over the surface finish.
The accurate determination of the skin friction coefficient for a particular surface finish involves a complex set of measurements. For many approaches, the instrumentation is fragile, difficult to initially calibrate, difficult to install and/or keep consistently calibrated as test conditions change during a measurement. Further, the test fixtures used to determine a skin friction coefficient are often customized for a particular measurement. Thus, the test fixtures are redone from test to test. In view of the above, new methods and apparatus are needed for characterizing the skin friction coefficient of a surface finish on a material.